1. Field of the Invention
This invention pertains to detection of radiation, and more particularly to multispectral radiation detectors for detection of radiation in multiple spectral bands.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
It is desirable to employ radiation detectors to convert electromagnetic radiation, such as infrared (IR) radiation, into electrical signals. The term photodetector is sometimes used, and is used herein, to refer to any type of radiation detector, i.e. a detector that detects electromagnetic radiation. Such detectors may be used in a variety of applications, including thermal imaging and transmission of information using signals having infrared wavelengths. One type of photodetector is the junction photodetector, or photodiode, which has a semiconductor p-n junction that produces electrical current under illumination with electromagnetic radiation. Other approaches, such as the use of bolometers which detect temperature changes caused by incident radiation, or quantum-well intra-subband detectors in which incident radiation causes excitation of electrons between confined energy states of a quantum well, generally provide lower sensitivity and/or slower frequency response.
The operation of a basic semiconductor p-n junction photodetector is illustrated by the energy band diagrams of FIGS. 1A-B. An energy band diagram, showing electron energy vs. distance, of a p-n junction under equilibrium conditions is shown in FIG. 1A. This diagram includes conduction band edge 10, valence band edge 12, and Fermi energy level 14. The energy difference 16 between the conduction and valence band energies is known as the energy gap, or bandgap, of the semiconductor. Because the same semiconductor material is used throughout the junction of FIG. 1A, this energy gap is shown as constant with distance throughout the junction.
Conduction in a semiconductor can generally be described in terms of the movement of electrons in the conduction band (having energy at and above that of conduction band edge 10) and holes in the valence band (having energy at and below that of valence band edge 12). The proximity of Fermi level 14 to conduction band edge 10 on the left side of the junction indicates that this portion of the semiconductor is doped n-type, while the right side of the junction is doped p-type. On the n-type side of the junction the majority carriers are electrons and the minority carriers are holes, while the reverse is true on the p-type side. The p-n junction includes a built-in electric field xcex50 in the junction region where the conduction and valence band edges are bent. The field exerts a force moving any holes appearing in this junction region to the right (in the direction of the field, as shown by the arrow in FIG. 1A), and moving any electrons appearing in the junction region to the left (opposite the direction of the field).
A photodiode is typically operated with the p-n junction reverse-biased, as shown in FIG. 1B. As in the case of built-in electric field of FIG. 1A, the larger applied electric field of FIG. 1B forces electrons toward the n-type side of the junction and holes toward the p-type side. Electrons and holes may be generated in the junction region by absorption of an incident photon such as photon 18. If photon 18 has energy higher than energy gap 16, absorption of the photon may provide energy to excite an electron from the valence band to the conduction band, creating conduction electron 20 and hole 22. The junction region 24 over which electric field xcex5 appears may be considered the collection region of the photodiode (collecting the photogenerated carriers), while the outer neutral n-type and p-type regions may be considered absorber regions, as Well as contact regions for connecting the photodiode to a surrounding circuit. Because electrons are collected on the n-type side and holes on the p-type side, each contact collects photogenerated majority carriers. The designation of a xe2x80x9cmajorityxe2x80x9d or xe2x80x9cminorityxe2x80x9d carrier is dependent upon the location of the carrier within the device. A hole formed by absorption of a photon on the n-type side of the photodiode is a minority carrier when formed, and becomes a majority carrier upon being transported by the electric field to the p-type side of the photodiode.
When properly biased, the photodiode thus produces a current related in a known manner to the electromagnetic radiation incident thereon. Photodiodes are used, for example, to detect short-, mid-, and long-wavelength IR radiation having wavelengths from about 1 xcexcm to about 30 xcexcm.
Semiconductor-based IR photon detectors (photodiodes), as well as other types of radiation detectors, are generally characterized as having an energy gap (bandgap) that is suitable for absorbing radiation within a specified spectral region. Infrared detectors that gather data in more than one IR spectral band can determine increased information from the scene to further improve sensitivity above that of single-band detection. Because a new dimension of contrast is obtained, the detection of radiation within two or more spectral regimes using a single detector has been established as a desirable goal. A single detector capable of detecting radiation of two or more distinct spectral regions, or xe2x80x9ccolors,xe2x80x9d may be referred to as a multispectral, or multicolor, photodetector.
There are two primary ways to achieve multicolor capability: xe2x80x9cmultiple detectorxe2x80x9d systems having separate detectors for each spectral band; and xe2x80x9cmultispectral detectorsxe2x80x9d that provide separate but spatially and temporally collocated signals from multiple IR spectral bands using a single detector element. To obtain multiple spectral band sensitivity, multiple detector systems currently rely on cumbersome imaging techniques that either disperse the optical signal across multiple IR detectors or use a filter wheel to spectrally discriminate the image focused on single detecting element. In comparison, integrated multispectral detectors offer separate and simultaneous sensitivity to different spectral bands within the same detector unit cell (spatially collocated). The use of an integrated multispectral detector eliminates the need for aligning two or more detectors and also reduces the number of on-board optical components, thereby providing significant reduction of weight and power in a simpler, more reliable, and less costly package. Furthermore, the temporal and spatial co-registration between each spectral field occurs on the pixel level, which enables high-performance signal processing.
Referring now to FIG. 2A, there is shown a perspective view of the wafer layer structure of an exemplary single-color photodetector 200. Such a photodiode detector is formed from a plurality of absorption layers stacked on a common substrate. To fabricate such a photodetector, an n-type absorption layer 212 is grown on a transparent substrate 211, followed by a p-type barrier layer 213 to form the IR photodiode. Light 201 within the spectral absorption band 212 of the n-type absorber layer which impinging upon the photodiode of detector 200 is transmitted through transparent substrate 211, and absorbed by n-type absorber region 212. When the photodiode of the junction of layers 213, 212 is properly biased, the absorption of the light causes a corresponding current to flow across the p-n junction of the photodiode. Photodetector 200 thus produces a current having a magnitude related in a known manner to the intensity of radiation within the spectral band of region 212 impinging on the photodiode of detector 200.
Two-color detection is achieved by extending the structure of photodetector 200 by one additional absorbing layer tuned to respond to the IR radiation that transmits through the lower layers. Referring now to FIG. 2B, there is shown a perspective view of the wafer layer structure of an exemplary two-color photodetector 250, which is also formed from a plurality of absorption layers stacked on a common substrate, and as described in U.S. Pat. No. 5,113,076 (Schulte), the entirety of which is incorporated by reference herein. Two-color detectors such as photodetector 250 extend upon single-color design of FIG. 2A by simply growing a second n-type absorbing layer 214 on top of the p-type barrier layer 213 to form two back-to-back (tandem) photodiode junctions.
Vertically stacking two p-n junctions, as illustrated in FIG. 2B, permits incorporation of two tandem photodiodes or detectors (layers 214, 213; 212, 213) into a single detector element 250. Varying the composition of the absorbing materials of n-type absorption layers 212, 214 controls the spectral sensitivity and cutoff wavelength of each junction. The first layer 212 responds to a shorter wavelength IR radiation (i.e., light 201), allowing the longer wavelength radiation (i.e., light 202) to pass through the first absorber and barrier layer and to be absorbed in the longer wavelength-sensitive material of layer 214 on the top. An SLS absorption layer of a photodiode generally absorbs radiation up to a cutoff wavelength. It is in this sense a long-pass filter, passing (not absorbing) radiation above its cutoff wavelength. Radiation below its cutoff wavelength is absorbed, meaning the photons of the EM radiation are converted to electrons, which result in a current delta across the photodiode junction when the photodiode is properly biased. The amount of current resulting from a given intensity of in-band radiation depends on the absorption coefficient of the photodiode, as will be appreciated.
While material layer design dictates the spectral bands of the respective n-type absorption layers, the fabrication process determines the detection mode. Two-color detectors fabricated from multi-layer materials, such as detector 250, can operate in either sequential or simultaneous mode. In simultaneous mode, each detector (e.g., a unit cell of an array of detectors, where each cell represents a pixel) is independently accessed by making electrical contact not only to the top and bottom (common) n-type absorber layers, but also to the shared p-type barrier layer. For simultaneous detection, two top contacts per unit cell are required, in addition to the bottom (common) contact. Thus, a first metal contact (e.g. an indium bump) is deposited on n-type absorption layer 214. A portion of n-type absorption layer 214 is etched to permit the deposition of a second metal contact onto p-type barrier layer 213. The two photodiodes may thus be independently biased, and the current in each independently sensed, to simultaneously detect the intensity of radiation absorbed by each of layers 212, 214. The spectral bands to which layers 212, 214 are sensitive may be referred to as bands 1 and 2, respectively. Discussion of these types of detectors can be found in R. D. Rajavel, J. L. Johnson et al., xe2x80x9cMolecular beam epitaxial growth and performance of HgCdTe-based simultaneous mode two-color detectors,xe2x80x9d J. Electr. Mat., vol. 27, no. 6 (1998): 747.
Referring now to FIG. 3, there is shown a perspective view of unit cells of a two-color photodetector array, in which each two-color photodetector 300 is operated in sequential detection mode. In sequential mode, the dual p-n junctions of detector 300 are electrically isolated (other than a common ground) from those of other unit cells in the same wafer by the same single-mesa delineation step. These form photodiodes electrically coupled together in series, illustrated schematically in FIG. 3 as diodes 322, 321. However, unlike in simultaneous mode, contact is made only to the top n-type layer 312 and to the bottom contact layer 311, which is used as an array common. Having no contact to the p-type barrier layer 313, each detector is sequentially accessed by the bias voltage applied across the tandem junctions. For example, a first bias voltage is applied across the two contacts, to activate the first photodiode 321 and effectively short photodiode 321; the opposite occurs in a second measuring operation using a second bias voltage. Sequential mode thus requires only a single metal contact (e.g., indium bump) per unit cell. Waveguiding of incident radiation may be employed to achieve 100% optical fill factor in both bands. For example, the angle of the mesa sides of each unit cell can be selected so that radiation does not pass through the gap between unit cells, but instead is reflected off he inside of a unit cell""s mesa walls so that it is detected by the photodiodes of the unit cell. Further discussion of these types of detectors may be found in T. De Lyon, J. L. Johnson, et al., xe2x80x9cMolecular Beam epitaxial growth of HgCdTe midwave infrared multispectral detectors,xe2x80x9d J. Vac. Sc Technol. B, 16(3) pp 1321-1325, 1998.
Other multispectral detector designs also use a plurality of n-type absorbing layers stacked on a common substrate but the deposition of the p-type contact layers is omitted. Instead, in this approach, the photovoltaic junctions are created by implanting a p-type dopant into delineated areas of the absorbing layers at a later stage of fabrication. Such multispectral detectors are described in U.S. Pat. No. 5,479,032 (Forrest et al.), the entirety of which is incorporated by reference herein.
Unfortunately, there are fundamental material issues associated with the growth of such multispectral detectors employing a plurality of stacked absorbing layers. These structures consist of absorption layers traditionally formed from semiconductor alloys. In each of the layers, the composition of the alloy is chosen to provide an energy gap sensitive to the spectral band of interest. As the layers are stacked (grown), the composition is changed toward decreasing energy gap; further, each absorbing layer must be optically thick enough to absorb the radiation and provide spectral discrimination of the incident light. However, with the change of composition and energy gap comes a substantial change in lattice parameter relative to the substrate and/or the preceding layer. As a result, lattice mismatch dislocations can propagate up though successive absorption layers, diminishing the crystalline quality of the material and ultimately leading to reduced performance.
Typically, a lattice mismatch of xcex94a/a0 less than 0.05% is desirable (where a0 is the lattice constant of the substrate or base layer on which an epi layer is grown, and xcex94a is the difference in lattice constant between the epi layer and the substrate, or preceding layer, on which it is grown). Requirements for lattice matching can either restrict the alloy composition to a small subset of available possibilities, or the limit the growth thickness of the absorbing layer, which may prevent adequate absorption of the light in the respective spectral band. Forest et al., in the ""032 patent, circumvented this problem by including buffer layers of increased lattice constant interlaced between the stacked absorbing layers. This approach complicates the growth; also, it does not completely prevent lattice mismatch dislocations from forming but merely getters or concentrates them in regions outside the active absorption layers. Other approaches rely on the very small lattice mismatch associated with closely lattice-matched ternary and quaternary compounds such as AlGaAs, HgCdTe, or InGaAsP. This approach restricts the choices of available alloy. Hence, the diversity of spectral bands that can be combined in a given detector stack is limited to but a few selections afforded by nature.
There is, therefore, a need for improved multispectral photodetectors.